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RC7108
PRODUCT SPECIFICATION
6
Serial Data Interface
The RC7108 features a two-pin, serial data interface that can
be used to configure internal register settings that control
particular device functions. Upon power-up, the RC7108 ini-
tializes with default register settings. Therefore, the use of
this serial data interface is optional. The serial interface is
write-only (to the clock chip) and is the dedicated function of
device pins SDATA and SCLK. In motherboard applications,
SDATA and SCLK are typically driven by two logic outputs
of the chipset. Clock device register changes are normally
made upon system initialization, if any are required. The
interface can also be used during system
operation for power management functions. Table 2
summarizes the control functions of the serial data interface.
Table 2. Serial Data Interface Control Functions Summary
Control Function
Clock Output
Disable
Description
Any individual clock output(s) can be
disabled. Disabled outputs are actively
held low.
Provides CPU/PCI frequency selections
beyond the 133MHz provided upon power-
on. Frequency is changed in a smooth and
controlled fashion.
Puts all clock outputs into a high
impedance state.
All clock outputs toggle in relation to X1
input, internal PLL is bypassed. Refer to
Table 4.
Reserved function for future device
revision or production device testing.
Common Application
Unused outputs are disabled to reduce EMI
and system power. Examples are clock
outputs to unused PCI slots.
For alternate microprocessors and power
management options. Smooth frequency
transition allows CPU frequency change
under normal system operation.
Production PCB testing.
CPU Clock
Frequency Selection
Output Tristate
Test Mode
Production PCB testing.
Reserved
No user application. Register bit must be
written as 0.